For optimal performance, the frequency content of relatively high frequency communication signal processing systems, such as those used for generating wideband chirps for synthetic aperture radar, should be as pure as possible, in particular, they should exhibit phase continuity or coherency through the entire output frequency range. Analog synthesizer-based systems, which offer a relatively wide tuning range, suffer from arbitrary phase steps when switching between local oscillators. A direct digital synthesizer (DDS), on the other hand, provides phase continuity with low noise when switching, but is capable of operation within a relatively narrow tuning range (e.g., 100 MHz).
One technique currently used to generate a wideband chirp involves multiplying up the output chirp. of a DDS so as to realize the desired output frequency range of the system. Unfortunately, successive multiplications also multiply spurious noise by the same factor. This problem is compounded by the fact that radiation requirements customarily limit the choice of DDS to those having relatively low frequency rates, which means that even higher multiplication factors are required. Another approach, which is not necessarily acceptable, is to limit the frequency range (width) of the chirp and use receiver processing to resolve phase errors associated with the discontinuities.